In fact, each time you share a content, it says that you are responsible of it. Since you will join servers hosted by other players, it should be your responsibility to follow the rules here. But without private restrictions, it is more like a public place. If you display nudity pictures in a public place, you are responsible of it. Moreover, it is an international place. So, if you would like to follow/respect the rules of these countries, I suppose that you should not put nudity on the cars.

The rule of the thumb to know if it is right is: would you do it in a international restaurant with kids?

It is not a clear and strong rule that everybody respect, but I believe it is the right spirit to have.